Remove unused factor levels in a data frame

dropall.R

dropall <- function(x){
    isFac = NULL
    for (i in 1:dim(x)[2]){isFac[i] = is.factor(x[ , i])}

    for (i in 1:length(isFac)){
        x[, i] = x[, i][ , drop = TRUE]
        }
    return(x)
    }